As is known, there are today available numerous electronic devices made at least in part of silicon carbide (SiC).
For example, there are available today metal-oxide semiconductor field-effect transistors (MOSFETs) made at least in part of silicon carbide, which is characterized by a bandgap that is wider than the bandgap of silicon, and hence also by a critical electrical field greater than the critical electrical field of silicon. In fact, typically the critical electrical field of silicon carbide is in the range between approximately 100 V/μm and 400 V/μm, whereas the critical electrical field of silicon is in the range between approximately 20 V/μm and 50 V/μm.
Thanks to its high critical electrical field, silicon carbide enables provision of junctions having breakdown voltages higher than what may be obtained using silicon. Consequently, the use of silicon carbide enables provision of MOSFETs having levels of doping higher than traditional silicon transistors. Furthermore, said MOSFETs may be formed by regions having thicknesses smaller than traditional silicon transistors, and hence are characterized by low on resistances (Ron).
On the other hand, silicon carbide has a low diffusiveness of the dopant species, even at high temperatures; moreover, as compared to silicon, silicon carbide is characterized by a low (surface) mobility μ of the carriers, which typically does not exceed 50 cm2/Vs. In turn, the low mobility μ of the carriers limits to a certain extent the possibility of obtaining even lower on-resistances.
In order to combine the advantages of silicon and silicon carbide, semiconductor devices have been proposed made both of silicon and of silicon carbide. In this connection, U.S. Pat. No. 5,877,515, which is incorporated by reference, describes a semiconductor device having an epitaxial silicon layer, which is deposited on a silicon-carbide layer, which in turn is deposited on a silicon substrate.
In practice, the silicon-carbide layer enables a concentration of charge to be obtained greater than what may be obtained in the case of a silicon layer, given the same breakdown voltage. However, it is possible that in certain conditions, and in particular in the case where the semiconductor device is biased so as to work in a region of inhibition, a non-negligible electrical field is generated within the epitaxial silicon layer. In said conditions, it is the silicon itself that limits, with its own critical electrical field, the breakdown voltage of the semiconductor device.